# How long does it take for the Magnetic Field to reach the detector?

• LitleBang

#### LitleBang

Suppose I had an electromagnet that would instantly go to full power when turned on (impossible) and at one light second away a detector capable of detecting the electromagnet. When I turn the electromagnet on how long does it take for the field to reach the detector?

One second.

One second.
I am not sure about that because the velocity you are assuming is that of an EM wave. A pure magnetic field does not transfer energy, it just puts it into storage, so I don't think we can measure a propagation delay.

I am not sure about that because the velocity you are assuming is that of an EM wave. A pure magnetic field does not transfer energy, it just puts it into storage, so I don't think we can measure a propagation delay.
1 s is correct. You cannot switch on a “pure magnetic field”. That action generates an E field too.

davenn
he velocity you are assuming is that of an EM wave.

Please don't tell me what I am assuming. (Unless you are claiming ESP!) That's what you get using retarded potentials.

so I don't think we can measure a propagation delay

If you have a magnet and a compass, you most certainly measure the difference in time between energizing the magnet and the motion of the compass.

If you cycle the thing on and off a few hundred thousand times a second, it's a radio, isn't it? Probably not a good one.

anorlunda
If you cycle the thing on and off a few hundred thousand times a second, it's a radio, isn't it? Probably not a good one.
Yes. In fact a very similar thing happens in MRI. The coils used to transmit RF energy in MRI are actually not designed to transmit in the far field, but predominantly send energy to the local magnetic field, the near field. You can certainly detect far field signals, but as you say they are rather inefficient since most of the energy gets reflected back into the amplifier.

Interesting, a connection to the propagation of light. I can't post new ideas but I can post a question that leads to new ideas. How very very interesting.

The idea that electromagnetic field changes propagate at the speed of light isn't exactly new...

davenn
Interesting, a connection to the propagation of light. I can't post new ideas but I can post a question that leads to new ideas. How very very interesting.
As pointed out this is not a new idea. Electromagnetic waves (which is what light is composed of) carry information about changes in the electromagnetic field.

Interesting, a connection to the propagation of light.
The history is interesting - here's the very oversimplified version:

That light propagated at a speed of about ##3\times{10}^8## meters/second was known since early in the 18th century (although not necessarily using those units, and ever more accurate measurements have been made since then).
During the 1860s James Maxwell formulated his equations describing the origin and behavior of electrical and magnetic fields, and discovered that these equations predict that changes in the electrical and magnetic fields would produce waves. He calculated the predicted speed of these waves... and the answer turned out to be ##3\times{10}^8## meters/second.
He then somewhat boldly suggested that these waves were light, that light was the electromagnetic radiation that his theory predicted. He was right.

So there is indeed a connection between the propagation of light and the propagation of your magnetic field changes - they are one and the same thing.

Redbelly98, vanhees71 and Ibix
He calculated the predicted speed of these waves... and the answer turned out to be ##3\times{10}^8## meters/second.

I have wondered how Maxwell must have felt when he first did that calculation and realized the connection.